Fibers, comprised of materials, such as glass graphite, boron, etc., have heretofore been used as a filler to reinforce polymers, such as polypropylene, polyethylene, polystyrene, etc., to form composite structures. Typically, such fibers are formed in short, straight linear strands. Fiber reinforcement is an excellent way to increase polymer strength without destroying the weight and manufacturing advantages of plastics over other material, such as, wood, metals and ceramics.
Composite structures made of conventional short-fiber fillers suffer from several problems which occur as the result of the unidimensional nature of the filler. Since the fibers are essentially one-dimensional, the fibers tend to align in one direction in the mold, creating an anisotropic composite. In other words, the elastic properties of the composite are not the same in all directions. Because of this, the anisotropic composite has lower strength perpendicular to the plastic flow in the mold than in the direction of flow. It also has low strength at the weld lines where the fibers align and lose much of their reinforcing effect, thereby resulting in low predictability of properties. Efforts to prevent or minimize these problems, in the main, have been directed at techniques for preventing straight fiber alignment using specialized molds or creating magnetic fields to orient the fibers.
An exception to the above generality is the study by Y. Kagawa et al., reported in "Some Properties of Composite Metals Reinforced with Helical Fiber", Composite Materials Ed., Proc. Japan - U.S. Conference, Tokyo 1981. Kagawa studied the reinforcing properties of a single, large diameter (150 microns), continuous, helical tungsten fiber for a copper matrix composite.